Touch screens function as both input and output tools for a variety of computerized devices, including general purpose computers as well as specialized machines. As input devices, they allow computer users to provide input to their computers by touching screens, either manually, with light pens, with magnetic styli, or with other objects. As output devices, they provide computer users with information in video form. These functional descriptions characterize most, if not all, touch screens; however, existing touch screens vary considerably in their underlying technologies.
While a variety of alternative touch screen technologies is currently available, available alternatives are not ideal for all applications. In particular, one family of touch screen applications that is not adequately supported by currently available underlying technologies is large desktop or work-surface touch screens. Such touch screens could provide tools useful for operating many existing computer graphics applications. For example, large, detailed drawings and schematics generated by architectural, mechanical engineering, and electrical engineering software could be more easily viewed and modified using desk-sized touch screens than using smaller, more conventional, monitor-based touch screens. Similarly, desk-sized touch screens would be useful in complex control panel applications, such as audio mixing consoles and industrial plant control panels. Desk-sized touch screens would be useful in computerized public-access information kiosks. Finally, large touch screens for office applications such as word processing and spreadsheets that run on windowing operating systems would be useful in many settings.
Conventional technologies fail to support large desktop or work-surface touch screens in two ways. First, the technologies underlying the output components of touch screens have previously made large output displays impracticable for many applications. Traditional CRT and LCD output displays cannot easily be scaled up to screen sizes with greater than 30" diagonals. Second, the touch sensor technologies underlying the input components of touch screens have heretofore been unable to provide large work screens capable of simultaneously detecting multiple touches, capable of operating without interference while stacks of books, telephones, and other common work-surface objects are resting on them, capable of simultaneously functioning as both the touch surface for receiving input and the display surface for providing output, and also capable of providing these characteristics in a form that does not require the work screen surface to be part of a large prism or other bulky, heavy, expensive, or otherwise impracticable component.
Large output displays can now be provided by digital micromirror technology. Digital micromirror projection displays can easily be scaled up to desk-sized screens without becoming excessively bulky or heavy, without dramatically increasing cost, and without becoming otherwise impracticable. Furthermore, video display technologies other than micromirror projection displays that increase the feasibility of large work-surface touch screens might develop in the future. Projection display technologies now make the output components of large, desk-sized touch screens practicable; however, existing touch sensor technologies fail to make the input components of large touch screens practicable in one or more of a number of respects.
Existing touch screens incorporate a number of alternative technologies for sensing touches. These technologies include at least the following variations: (1) touch screens comprised of layers of clear flexible plastic and of sensors that detect changes in conductivity or capacitance when pressure is applied to the layers of plastic; (2) touch screens comprised of sensors that detect interruptions in surface acoustic wave or linear LED arrays when screens are touched; (3) touch screens comprised of sensors that detect touches by light pens or magnetic styli; (4) touch screens comprised of photodiodes that respond to totally internally reflected light generated within the screens when CRT raster signals scan over touch points on screen surfaces and of devices that determine precise raster positions for times when photodiode responses are detected; (5) touch screens comprised of prisms within which light from a light source is totally internally reflected, and of cameras positioned to only detect light totally internally reflected, such that when the screen surface is touched, light from the light source is no longer totally internally reflected into view of the camera; and (6) touch screens comprised of panels through which ambient light reaches cameras such that the cameras can detect shadows where the panels are touched.
Pressure-sensitive touch screens, surface acoustic wave touch screens, and linear LED array touch screens are generally impracticable for use where simultaneous detection of multiple touch points is desired. These types of touch sensors also cannot operate when stacks of books or other objects are left sitting on portions of touch screen surfaces. Both the capability of simultaneously detecting multiple touches and the capability of operating with objects sitting on the screen surface are desirable in a desk-surface touch screen.
Touch screens that detect touches by magnetic styli or light pens require users to make their touches with a stylus or light pen and often are sensitive to how such stylus or light pen is oriented relative to the screen when touches are made. Styli and light pens can also become misplaced and are generally less convenient to use than are users' fingers. In addition, light pens generally depend upon CRT raster signals for their operation, and thus are not readily adapted for use with micromirror projection displays. As noted above, CRT output devices are not easily scaled up to desk-sized screens.
Touch screens that rely upon the timing of CRT raster signals are generally limited to uses that incorporate CRT output devices and are not ideal for use with projection displays that do not incorporate CRT devices. As discussed above, CRT screens cannot readily be scaled-up to desk-surface sizes. An example of a touch screen that relies upon the timing of CRT raster signals is the device disclosed in U.S. Pat. No. 4,346,376 issued to Mallos, which is a device in which light from a raster signal is totally internally reflected within a screen panel with substantially parallel surfaces when a finger or other object touches the outer screen surface. The touch of a finger or other object on the outer screen surface causes light to reflect in a variety of directions within the screen panel, some of which are at angles conducive to total internal reflection. By identifying the precise time when photodiodes detect total internal reflection and matching that time to a known raster signal position for that time, the Mallos device identifies touch positions.
Prisms, such as those disclosed in U.S. Pat. No. 4,561,017 issued to Greene, can provide most of the performance characteristics desirable in a desk-sized touch screen; however, a desk-sized prism filled with water or oil would be heavy, expensive, and impracticable. Greene's device provides a prism into which light from a light source enters through a face not substantially parallel to the touch surface and totally internally reflects from the touch surface face of the prism; and a camera positioned to capture only light that has been totally internally reflected from the touch surface of the prism. When the touch surface of the prism is touched, light rays from the light source that previously reflected internally off the touch surface of the prism no longer are reflected. The interruption in total internal reflection is a result of the object used to make the touch displacing the layer of ambient air above the touch screen adjacent to the touch point. When the layer of air is present (i.e. when the surface is not being touched), rays of light internally reflect off the touch surface and into the view of the camera as a result of the angle at which the rays strike the touch surface and of the difference in the indices of refraction for the air and for the prism material (the index for air is significantly lower). When the air is displaced by a touch, the difference in the indices of refraction is significantly reduced, and light rays striking the screen surface at the touch points no longer are totally internally reflected into the view of the camera. Greene's device requires a large prism so that only internally reflected light from the light source reaches the camera; a touch screen comprised of a sheet of transparent material would not be compatible with Greene's invention because such a sheet would permit ambient light as well as totally internally reflected light to reach the camera. Greene's device also requires the use of a prism so that total internal reflection will occur at all, since total internal reflection of light originating from outside a sheet of transparent material having substantially parallel top and bottom surfaces ordinarily will not occur within such a sheet. In other words, Greene facilitates the occurrence of total internal reflection by providing a prism with a face not substantially parallel to the touch surface through which light from a light source can enter.
Devices that comprise panels through which ambient light reaches cameras such that touching the panels creates shadows that are imaged by the cameras, such as the device disclosed in U.S. Pat. No. 5,483,261 issued to Yasutake, are limited in two ways. First, such devices rely upon at least some ambient light for their operation. Second, such devices are not readily adapted to uses where the presence of a touch must be an all-or-nothing proposition, since shadows created when the touch object nears the touch screen but before an actual touch is made are difficult to distinguish from those created when an actual touch is made.
Accordingly, it is an object of the present invention to provide a touch sensor apparatus than can readily be scaled up to a desk-sized work surface. It is also an object of the present invention to provide a touch sensor apparatus that can simultaneously detect multiple touch points and can continue to operate when objects are left resting on a part of the touch surface. Another object of the present invention is to provide a touch sensor apparatus whose touch screen panel can also be used as an output screen for a projection display. A further object of the present invention is to provide a touch sensor apparatus in which the touch screen panel is not integrated into a large prism or other bulky configuration. Yet another object of the present invention is to provide a touch sensor apparatus that senses touches only when actual contact by a finger-sized object is made with the touch screen touch surface. Still yet another object of the present invention is to provide a touch sensor apparatus that can operate regardless of the level of ambient light or without ambient light.